Optically addressed photoconductive light valves or spatial light modulators (SLMs) enable spatial modulation of light beams incident to them. Spatial modulation of a light beam entails changing the optical properties of the light valve, such as the intensity or phase of the light wave. Popular examples of an SLM are liquid crystal displays (LCDs), which convert image data stored in an electronic medium into a visible picture or an image. Spatial modulation of light incident on an LCD is performed by producing changes in the optical properties of a liquid crystal layer in response to an array of applied electrical voltages, the values of which can vary in time to produce a moving picture or otherwise alter the image. These changes are in the prior art introduced locally by supplying potential differences at different points across the liquid crystal layer by means of an electrode array system in contact with or proximal to a liquid crystal layer.
An alternative approach to performing spatial light modulation that eliminates the expense of the electrode array light modulation is represented by the technology implemented in optically addressed light valves or SLMs. The optically addressed SLM responds to an optical signal that causes local changes in optical properties of an electro-optic material, e.g., a layer of liquid crystal, to produce an output image. Prior art optically addressed spatial light modulators contain a liquid crystal or other electro-optic material layer and a photoconductive layer formed usually of semiconductor material. The semiconductor material for the photoconductors in prior art light valves have been selected from a variety of materials that absorb light in at least a portion of the range (400 nm-700 nm) of visible wavelengths and include, for example, single crystal silicon (Si), amorphous Si, amorphous silicon carbide, gallium arsenide (GaAs), Bi12SiO20, zinc sulfide (ZnS), and cadmium sulfide (CdS). Almost all of these prior art structures operate in a reflection mode so that the visible readout or illumination light that is modulated by the SLM does not reach the photoconductive layer because at least a portion of the visible spectrum would be heavily absorbed in the photoconductive layer and a good full color image could not be obtained.
There are advantages for the optical engine of a display system to be able to operate the light valves in the transmissive mode. Several devices that use near infrared “read light” have been developed. Monochromatic transmissive displays that use red read light and green or blue write light have been demonstrated with ferroelectric liquid crystals. Ferroelectric liquid crystals are inherently bistable, which makes the attainment of a large range of intensity or gray scale difficult. For a large image, full color display with a fine gray scale, there has long been a need for an optically addressed spatial light modulator that operates in a transmissive mode with a non-ferroelectric liquid crystal and is capable of modulating substantially all of the spectrum of visible light to form an image. An advantage of such a spatial light modulator is that projector light engines using transmissive light valves are cheaper and simpler to make.